In recent years, a WDM optical communication system, in which plural optical signals with different wavelengths are multiplexed and transmitted through the same optical transmission line, have being vigorously developed as means for epochally increasing the transmission capacity of the optical transmission line.
In order to extend the transmission distance of the WDM optical communication system, it is indispensable that the levels of the transmitting optical signals are high and equalized, and strenuous efforts have been made to meet the aforementioned requirement. For example, in one of approaches to the subject, optical variable attenuators are respectively connected with the output ports of the signal light sources, and the levels of the optical signals are equalized by adjusting the optical variable attenuators, but the high transmitting powers cannot be achieved. In the other approach, optical fiber amplifiers are respectively connected with the output ports of the signal light sources, and the levels of the optical signals are equalized by adjusting the gains of the optical fiber amplifiers. According to this system, the problems mentioned in the beginning can be solved, but the system is magnified and high-priced. Then, it is extremely desirable to decrease the number of the optical fiber amplifiers.